Conventionally, there are many known techniques for converting a document into electronic image data using a scanner and for subsequently recognizing characters in the data. In the implementation of these conventional techniques, it is often the case that the characters are black and the background is white. Moreover, because a scanner is used, the illumination is bright and does not have an unevenness, and, hence, a high-definition image can be obtained. Consequently, pixels constituting characters are likely to be dark, pixels constituting the background are likely to be bright, and the characters and the background can be separated by thresholding.
Recently, there is an emerging technology directed to the recognition of recognizing characters on signboards, indicators, etc. in a scene image captured by a digital camera. In this case, however, the illumination condition is unknown, and the colors of the characters and the background are diversified. Therefore, it is not possible to set a threshold for pixel values used for separating the characters and the background in advance, and the threshold may vary depending on the location. For this reason, separation of the characters from the background has been difficult.
For example, in a technique described a paper by Paul Clark and Majid Mirmehdi, entitled “Location and recovery of text on oriented surfaces,” in SPIE conference on Document Recognition and Retrieval VII, pp. 267-277, The International Society for Optical Engineering, January 2000, on the assumption that Text Planes such as sheets of paper, signs, etc., on which characters are printed are rectangles, the Text Plane is detected automatically, and characters are detected. In this detection, the illumination on the Text Plane is assumed to be even. However, in actual practice, the real condition is not as set by the assumption, and there are many cases in which this method fails to produce satisfactory results because of illumination unevenness on the Text Plane.
[Non-Patent Document 1]
Paul Clark and Majid Mirmehdi, “Location and recovery of text on oriented surfaces,” in SPIE conference on Document Recognition and Retrieval VII, pp. 267-277, The International Society for Optical Engineering, January 2000.